1. Field of the Invention
This invention relates to light valves, and more particularly, to a light valve with rotatable vanes controlled by separate integral servo motors that provide a fast response time for the light valve. One use of the light valve is in high speed film printing. The invention is directed to a shutter control system having improved zero-light position control.
2. Description of the Prior Art
A light valve is a device that varies the size of an aperture through which a light beam passes. A common use of light valves today is in film printing. The light valve is adjusted to control the cross-sectional size of the light beam passing through it for adjusting the exposure in accordance with the light levels of scene changes in the film being printed. Typically, separate light valves are used for each of three principal colors in the film printing process (red, green and blue), and the exposures from each light valve are independently controlled.
A common form of light valve includes a pair of vanes carried on a pair of rotating shafts. The vanes protrude into the path of the light beam, and the shafts are rotated to move the vanes toward or away from each other to form a variable width optical slit for controlling the amount of light passing through the vanes. Usually, the shafts are counter-rotated relative to each other and through approximately the same phase angle so the center of the aperture formed by the vanes does not appreciably shift laterally during operation of the light valve.
Light valves of various forms have been developed in the past for controlling the phase angle of the shafts that rotate the vanes. These have included use of a stepper motor, or means for rotating the shafts against stops which have been preset prior to each new aperture change. When the shafts are rotated by a common motor, mechanical coupling means such as linkages, gears, or the like are used to rotate both shafts simultaneously through the same angle. Such mechanical coupling increases the inertia of the moving parts which, in turn, slows down the response time of the light valve. Also, it is difficult to avoid backlash and added elasticity which can cause oscillations in the mechanical system that controls the vanes.
Moreover, these problems are amplified when attempting to speed up the response time of the light valve. U.S. Pat. No. 4,594,539 discloses a light valve having a vane control system which can effectively operate at higher speeds than previously known light valves. For instance, in the past, the fastest light valves with mechanical coupling had a response time on the order of about 5 milliseconds. In a film printing system, "response time" is the time required for the vanes to assume a new position to produce a new sized aperture for each scene change in the film. Previous light valves with their connecting linkages and gearing experienced oscillation problems when operated at higher speeds. Film printing speed with such devices had been limited to about 600 feet per minute. The light valve disclosed in '539 patent fulfilled the need to provide a film printing system that could operate at higher speeds. The slow response time of the light valves with mechanical coupling as used in previous film printing systems had been a principal obstruction to higher speed printing.
In the '539 patent (see FIG. 8) the vanes 18 move in unison either toward or away from the light source. As the left vane rotates counter-clockwise toward the light source, the right vane rotates clockwise also toward the light. In normal operation there are instances in which the vanes may have to be totally closed instantaneously to completely block passage of light. In the FIG. 8 embodiment, the vanes do not close entirely. When moved to the closed position they leave a tiny narrow gap between their adjacent edges (through which one can see a narrow slit of light). The vanes must never touch each other because the power of each servo motor driving the vanes is approximately one horsepower; and the acceleration of the vanes is on the order of 1,000,000 radians/sec.sup.2. If the vanes were long enough to touch each other (to block all light transmission) they could irrecoverably jam or damage the vanes or the servo motors.
There are certain applications that require a light valve that closes completely so that no light is transmitted. The present invention solves the problem of closing the vanes, while operating at high speeds, so that the vanes and servo motors are not damaged in the zero-light position.